An electrical contact or contact is an important spare part between two conductors in a switch or a circuit allowing a current to pass through mutual contact, which bears the functions of connecting, carrying and disconnecting a normal current and a fault current. The quality and service life of the contact directly determines the quality and service life of the entire switch or circuit. The electrical contact or contact is mainly used in a relay, a contactor, an air switch, a current limiting switch, a motor protector, a microswitch, an instrument, a computer keyboard, a hand-held set, a household appliance, an automotive electrical appliance (a window switch, a rear-view mirror switch, a lamp switch, a starter motor and other load switch), a leakage protection switch, or the like. The electric contact or contact may be prepared of multiple materials, which mainly include silver, silver-nickel, silver-copper oxide, silver-cadmium oxide, silver-tin oxide, silver-tin oxide-indium oxide, silver-zinc oxide, red copper, brass, phosphor copper, bronze, tin-copper, beryllium copper, copper-nickel, zinc-cupro-nickel, stainless steel, or the like.
In automotive appliances, household appliances, computer keyboards, hand-held sets and other devices, switch components thereof are usually printed circuit boards (PCB) provided with contacts and provided with combinations of contacts and rubber keypads. A circular contact on the PCB is divided into two non-conducting halves by a straight line or curve (like an S-shaped curve, and an M-shaped curve). The contact on the keypad is a circle without splitting. A circuit on the PCB can be switched on by using a circular contact of the same diameter on the keypad to make a face-to-face contact with the circular contact on the PCB. The contact on the keypad is made of conductive rubber or metal. The conductive rubber has a larger contact resistance when being contacted with the contact of the PCB. The conductive rubber contact is not suitable for switching on a PCB circuit having a large current (such as current greater than 50 mA). The metal contact has a smaller contact resistance when being contacted with the contact of the PCB. The metal contact not only can be used to switch on a PCB circuit having a smaller current, but also can be used to switch on a PCB circuit having a larger current. However, the metal contact has the problems of unsatisfactory chemical corrosion resistance, unsatisfactory arc-erosion resistance and high production cost at present, thus limiting the applications thereof.
In the atmosphere, a switching element usually generates an electric spark or electric arc when switching on or switching off a circuit. The subsistence of the electric arc phenomenon of the switch will result in contact oxidation and ablation, and may carbonize organic matters in the air, thus producing carbon deposition, which gradually increases a contact resistance of the switch and even causes a circuit break of the switch.
A patent document with a patent application number of 201220499100.X discloses a “Three-layer Composite Electric Contact”, wherein the contact is provided with a layer of silver plated on a contact surface of a copper-based contact body, so that the contact has better electrical conductivity, and the production cost is saved than that of completing using silver to produce the contact. Although the electrical conductivity and heat conductivity of the silver are highest among all the metals, the silver has poorer atmosphere corrosion resistance and poorer salt-mist resistance. The silver is easily reacted with sulphuretted hydrogen (H2S) in the atmosphere to generate black silver sulfide. When the silver is used as a contact, although the primary surface resistance is smaller, the service life of the silver in the atmosphere is also limited. Although the cost of silver plating is lower than that of gold, the silver is still one of precious metals. Moreover, in such an electric contact, no rubber layer is provided; therefore, this electric contact is not suitable for performing heat vulcanization adhesion and heat vulcanization shaping with rubber to prepare a rubber keypad containing electric contact. Only a contact containing a rubber layer or a contact entirely made of conductive rubber can possibly perform heat vulcanization adhesion and heat vulcanization shaping with other rubber smoothly so as to prepare the rubber keypad containing electric contact, without causing quality problems like excessive glue and poor adhesion during heat vulcanization adhesion and heat vulcanization shaping.
A patent document with a patent application number of 200580045811.2 discloses a “Flat Primary Battery with Gold-plated Terminal Contact”, which may be applied to, for example, a digital camera. The battery may have a contact containing a lithium anode and a low resistance. The anode and a cathode may present a spirally-crimped sheet form with a baffle therebetween. External anode and cathode contacts are plated by gold so as to improve the contact resistance. Although the electric contact according to the present invention has small resistance, the performance thereof for resisting sparks produced by voltage is not ideal since a melting temperature of gold is poorer than that of tungsten, molybdenum and other refractory metals. Moreover, the expensive price of gold also limits the application range of the electric contact.
A patent document with a patent application number of 201020143455.6 discloses a “Nickel-plated Tungsten Contact”, which belongs to the technical field of basic appliance elements, and aims at solving the problem that the existing tungsten contact is easy to be oxidized to affect the electrical conductivity. In the prior art, the existing tungsten contact is mainly prepared by using pure copper as a solder to perform fusion welding on a rivet type seated nail and a tungsten plate. In this patent, an outer surface of the tungsten contact welding on the seated nail and the tungsten plate is enclosed and connected with a nickel-plate layer as the nickel-plated tungsten contact. The nickel-plated tungsten contact has a simple and practical structure and stable electrical conductivity, is durable in use, and is applicable to cars, motorcycles, electric horn and other electrical appliances. The contact of the patent uses the tungsten plate plus the nickel-plated layer, while the arc ablation resistance of nickel is low, so that the contact is not suitable for a relatively harsh occasion needing a higher working current or voltage. Our test shows that the nickel is served as a switch contact connects or disconnects (switches on or off) with the gold-plated contact. At a room temperature, but when the working current is 300 mA, the switching time is about 4000, then the contact resistance of the switch is significantly increased, or even to completely disconnect the circuit.
A U.S. Pat. No. 4,019,910 discloses to prepare a electroless nickel alloy plating bath. The nickel alloys contains boron or phosphorus, and one or more metals selected from tin, tungsten, molybdenum or copper. The electroless plating bath contains an ester complex obtained by reacting inorganic acid with polyhydric acid or alcohol, such as diboron ester, tungstate ester or molybdate ester of glucoheptonic acid. The nickel alloy is mainly constituted by nickel, and the nickel content is generally within the range of about 60% to about 95% by weight. The alloy has excellent mechanical property and corrosion resistance, and some alloys such as phosphorus-containing nickel alloys, in particular nickel-phosphorus-tin-copper alloys, have non-magnetic or non-ferromagnetic property. The polymetallic nickel alloy disclosed by the invention contains a relatively high content of boron or phosphorus. In the case of using as a contact material, the relatively large amount of boron or phosphorus will affect the initial resistance of the contact. Our tests show that, pure nickel, nickel alloy with high content of nickel (such as nickel-copper alloy or monel alloy, nickel-chromium alloy, etc.), nickel-containing stainless steel, or electroless nickel alloy using nickel as the main component, if serving as the contact of the switch, have poor arc resistance and low service life of switch.
A US patent application 20090088511 discloses an electroless plating solution used for selectively forming a cobalt-based alloy protective film on an exposed copper wire. The electroless plating solution includes a cobalt ion and another metal ion (tungsten and/or molybdenum), a chelating agent, a reducing agent, a specific surface active agent and a tetramethylammonium hydroxide. The use of the bath disclosed in this invention does not require the use of a copper seed layer (e.g., a palladium layer) prior to electroless plating. The protective film has the ability of anti-diffusion and anti-electromigration. However, this protective film, due to the high content of cobalt, is relatively hard and brittle. In addition, due to the arc, the cobalt-based alloy is very easy to produce oxides of cobalt and lead to increased surface resistance. The arc ablation resistance of this protective film is not good, so that this protective film is not suitable for manufacturing electrical contacts or contacts.
The invention with a U.S. Pat. No. 6,821,324 describes an aqueous bath for the chemical deposition of cobalt tungsten phosphorus containing cobalt chloride hexahydrate, soluble tungsten ion source from tungsten trioxide (WO3) or phosphotungstic acid [H3P (W3O10)4], and a phosphorus-containing reducing agent, free from alkali metal ions and alkaline earth metal ions, and the obtained deposited film is oxygen-free and has a low resistivity. Such deposited films can be used as capping layers or barrier layers in products such as semiconductor chips, very large scale integration (VLSI) products, jewelry, nuts and screws, magnetic materials, wings, advanced materials and automotive components to prevent interlayer metal diffusion and migration. A small variety of raw materials is selected for the plating bath described in this invention. Since the plating bath does not contain alkali metal ions and alkaline earth metal ions, the concentration of tungsten ions in the plating solution is low (particularly when tungsten trioxide is used as the raw material), the tungsten content in the formed cobalt-tungsten-phosphorus deposited film is difficult to be adjusted, and a deposited film having a high tungsten content is difficult to obtain. The bath described in this invention can be deposited on substrates such as silicon, silicon dioxide, jewels, magnetic materials and metals, without selectivity to the substrate. In addition, the temperature of the switching arc can reach 6000° C., while in the existence of oxygen, when being heated to above 300° C. cobalt is oxidized to produce CoO or Co3O4. The alloy with cobalt as the main component has poor arc ablation resistance, and is not suitable as a contact material, so few cobalt alloy electrical contacts or contacts are found in industry.
The invention with a U.S. Pat. No. 6,797,312 describes a plating solution containing no alkali metal is used for forming a cobalt-tungsten alloy. The plating solution can be formulated without the use of tetramethylammonium hydroxide. Prior to depositing cobalt-tungsten metal alloy onto the substrate a catalyst such as palladium catalyst is not used for pre-treating the substrate, and the plating solution can be used for obtaining the deposited cobalt-tungsten alloy layer. The cobalt-tungsten alloy contains a lot of cobalt element, not resisting switch arc ablation. The alloy of this invention also does not relate to how to carry out selective chemical deposition.
The invention with an application patent number 201110193369.5 of the inventor provides a “Pitted-surface metal and rubber composite conductive particle” which is formed by adhering a metal surface layer to a rubber matrix or slitting after adhesion. The metal surface layer is a pitted surface and has concave pits or convex points or both the two; the concave pits or convex points are formed on an outer surface, or an inner surface of the metal surface layer, or both the outer surface and the inner surface; the depths of the concave pits are smaller than the thickness of the metal surface layer; and the heights of the convex points are no less than one tenth of the thickness of the metal surface layer. The metal surface layer is made of metal or alloy, the outer surface can be plated with gold, silver, copper or nickel; the rubber base is silicone rubber or polyurethane rubber; a bonding layer may be between the metal surface layer and the rubber base, and the bonding layer is a heat curing adhesion agent, a primer or a material the same as the rubber base. Aids such as a coupling agent can be coated on the inner surface of the metal surface layer. The metal surface layer of the invention has high strength and stable conductivity of electricity, the adhesion layer has high strength, and the rubber matrix has sufficient elasticity. The invention does not provide a solution to the problems of arc ablation resistance of the conductive particles. The present invention also does not propose a specific method of obtaining one or more plated layers on the outer surface of the metal surface layer. In the present invention, the pitted skin is plated with precious metals such as gold and silver. Since the surface area is large, the amount of the precious metal is large and the cost is high.
It is well known that a melting point of tungsten in all pure metals is the maximum of 3410° C. A vapor pressure of Tungsten is very low, and an evaporation rate is relatively slow. A chemical property of tungsten is very stable. Tungsten does not react with the air and water at a room temperature. In case of not heating, hydrochloric acid, sulfuric acid, nitric acid, hydrofluoric acid and aqua regia in any concentration have no effect on tungsten. Alkaline solution also has no effect on tungsten. Tungsten is also a material having a relatively small resistivity and better electrical conductivity. In a variety of pure metals, the resistivity of tungsten is greater than silver, copper, gold, aluminum and molybdenum, but less than zinc, nickel, cadmium, palladium, iron, platinum, tin, lead, antimony, titanium, and mercury. Tungsten as the contact material is conducive to reducing the contact resistance of the contact. However, the hardness of the tungsten or tungsten alloy is very high, and it is difficult to obtain tungsten or tungsten alloy flake with a small thickness (particularly, a tungsten alloy flake having a thickness of smaller than 0.05 mm) by a mechanical pressing or powder metallurgic method. If the tungsten alloy flake is used directly in the production of metal contacts, the cost of the raw materials of the metal contacts will be increased, and it is difficult to cut or punch due to the high hardness of tungsten or tungsten alloy. Due to the significant difference between tungsten and other metal properties, there is no mature and widely used application technology in electronic products, especially in contacts.
The present invention will disclose an arc-ablation resistant tungsten alloy switch contact and preparation method thereof. Because such contact contains the rubber layer, the contact may perform heat vulcanization adhesion and heat vulcanization shaping with the rubber, thus preparing a rubber keypad having an arc-ablation resistant contact.